1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a hydrostatic continuously variable transmission in which a fixed capacity hydraulic pump and a variable capacity hydraulic motor are connected by a hydraulic closed circuit.
2. Description of the Relevant Art
Hydrostatic continuously variable transmissions are well-known. Such transmissions are applied to various vehicles, such as motorcycles. A hydrostatic continuously variable transmission is described in detail in Japanese Patent Laid-Open No. Hei 61-153057. In the hydrostatic continuously variable transmission, the constant-displacement swash plate hydraulic pump and the variable-displacement swash plate hydraulic motor are connected by the hydraulic closed circuit. The constant-displacement swash plate hydraulic pump is driven by the engine to produce hydraulic pressure. Power transmission is assured by applying the hydraulic pressure to a plunger on the variable displacement swash plate hydraulic mocor side, and also by changing the angle of the swash plate pressed against the plunger on the motor side.
Japanese Patent No. 2527199 describes a control method for a static hydraulic continuously variable transmission, in which an actual rotation speed (Ne) of a crankshaft, or the like, is compared with a target rotational speed (Ne), which has been determined in accordance with predetermined conditions, and an output is controlled by adjusting an inclination angle of a swash plate.
The prior art suffers several drawbacks. The transmission makes a noise because of its construction, under certain driving conditions. The noise occurs under the following two conditions: during forward downhill drive in REVERSE gears after uphill drive, and during backward downhill drive in FORWARD gears after an engine stop occurring during uphill drive (also during backward downhill drive in REVERSE gears after an engine stop occurring during uphill drive with gears in REVERSE).
Under the above-described driving conditions, the noise occurs when an excess play exists between the plunger and the swash plate. The excess play occurs because when the output shaft is turned reversely, a hydraulic pressure is built up in the hydraulic motor, and the amount of oil being delivered to the hydraulic motor side increases with an increase in the speed of reverse rotation. The hydraulic pump on the input side is being operated with the engine idling through a one-way clutch mounted between the hydrostatic continuously variable transmission and the engine. Therefore, if the speed of reverse rotation of the output shaft increases, the hydraulic pump remains operating at the idling speed. However, the hydraulic motor comes to require more oil, and consequently the hydraulic pump will fail to deliver a sufficient amount of oil to the hydraulic motor. Under these circumstances, the pressure to be applied to the plunger of the hydraulic pump will decrease, causing the excess play and noise to occur between the plunger and the swash plate.
In the case of normal rotation (e.g., during engine brake application on a downhill), the hydraulic pump speed also increases with an increase in the speed of rotation of the output shaft. Therefore, the specific amount of oil to be delivered from the hydraulic pump is insured, producing no noise. Therefore, it is desired to prevent the occurrence of a hydraulic pressure in the hydraulic motor for the purpose of noise prevention during such reverse rotation of the output shaft. Furthermore, it is undesirable to operate this type of hydraulic equipment in such a manner as to build up a hydraulic pressure in the hydraulic motor, when no power is being supplied to the hydraulic pump.
It is an object of the present invention to provide a variable transmission and a method of controlling a variable transmission to solve one or more of the drawbacks associated with the background art.
In a first aspect of the invention concerning the method of controlling the continuously variable transmission, the constant-displacement swash plate hydraulic pump and the variable-displacement swash plate hydraulic motor are connected by the hydraulic closed circuit, and the gear ratio is changed by changing the swash plate angle. The method of controlling the hydrostatic continuously variable transmission is characterized in that, when the output shaft of the continuously variable transmission is reversely turned by the turning force from the vehicle wheel, the angle of the swash plate on the hydraulic motor side is changed to change the gear ratio to a TOP side.
A second aspect of the invention is characterized in that the constant-displacement swash plate hydraulic pump and the swash plate hydraulic motor are connected by the hydraulic closed circuit, and the swash plate angle is changed to change the gear ratio. In the second aspect of the method of controlling the hydrostatic continuously variable transmission, when the engine power entering the continuously variable transmission is shut off, the swash plate angle on the hydraulic motor side is changed to change the gear ratio to the TOP side.
According to the first aspect, when the output shaft of the continuously variable transmission is reversely turned by the turning force from the vehicle wheel, the gear ratio is changed to the TOP side by changing the angle of the swash plate on the hydraulic motor side. In this state, the swash plate on the hydraulic motor side is at right angles with the axis of the output shaft, and therefore no hydraulic pressure is built up in the hydraulic motor, thereby preventing a noise occurrence. One advantage of the present invention is that this improvement can be realized by making a change to the method of control, and that additional structural components are not needed.
According to the second aspect, if the input of the engine power to the hydraulic pump side during an engine stop is interrupted, the swash plate on the hydraulic motor side is positioned to the TOP side, producing no hydraulic pressure on the hydraulic motor side. It is therefore possible to prevent an occurrence for this type of hydraulic equipment, wherein hydraulic pressure is built up in the hydraulic motor, when no power is being supplied to the hydraulic pump.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.